In 3.0, on WiredTiger, if you do a reverse natural order query on the oplog, it's possible that it returns only 1 document, even when the oplog has many entries. This can happen if the newest entry is visible, but the second newest entry is not yet visible. When we encounter a hidden entry in a reverse collection scan, we simply return EOF, as though we had hit the beginning of the collection. This can cause issues where rollback fails to complete and hits an fassert, since if the node rolling back does a reverse oplog query against its sync source and that cursor returns that there are no more entries, it assumes we hit the beginning of the sync source's oplog and thus that we have falling too far behind to recover.